Since the discovery of HTS materials (i.e. material that can retain its superconducting properties above the liquid nitrogen temperature of 77K) there have been efforts to develop various engineering applications using such HTS materials. In thin film superconductor devices and wires, most progress has been made with fabrication of devices utilizing an oxide superconductor including yttrium, barium, copper and oxygen in the well-known basic composition of YBa2CuO7-y (hereinafter referred to as Y123 or YBCO) which remains the preferred material for many applications, including cables, motors, generators, synchronous condensers, transformers, current limiters, and magnet systems for military, high energy physics, materials processing, transportation and medical uses. HTS wire based on these YBCO materials, commonly referred to as Coated Conductor or Second Generation (2G) wire is manufactured in continuous lengths of hundreds of meters or longer with critical current densities, Jc, of 3 MA/cm2 or higher at 77 K and self-field using roll-to-roll production lines.
Even though Y123 is the material of choice for HTS applications, these materials exhibit a suppression of Jc in magnetic fields, especially in fields applied along the crystallographic c-axis of the superconductor. This characteristic limits the applicability of these materials in applications that require operation in magnetic fields such as motors, transformers, generators, solenoids, accelerator magnets, energy storage magnets and MRI systems. Hence, it has been important to continue to improve the performance of Y123 superconductors, especially in fields applied in the c-axis direction. More importantly, it is important to achieve these improvements in a roll-to-roll process that is compatible with current wire manufacturing operations and produces a highly uniform and reproducible improvement along the length and width of the production length HTS wires.
One method to achieve such improvement includes “pinning” of superconducting vortices, which is thought to be the underlying mechanism for high critical current density, Jc, in HTS materials. To achieve pinning in HTS materials/wires, matching the local potential energy differences as closely as possible to the size of the normal core of the superconducting flux line or vortex have been attempted. It is generally thought that the cross-sectional core has a size on the order of the coherence length (which is several nanometers in HTS cuprates and grows with temperature). Moreover, it is thought that critical currents in polycrystalline HTS materials/wires are still further reduced by weak links at the grain boundaries, which are made worse by porosity, misalignment of the crystalline axis of adjacent grains, and by formation and accumulation of non-superconductor phases (compounds) at boundaries between superconducting grains.
There have been efforts to improve flux pinning of the RE123 superconducting materials. For example, the superconducting properties of YBa2Cu307-δ compounds with partial substitutions with europium (Eu), gadolinium (Gd), and samarium (Sm) were found to show an improvement in intra grain Jc (flux pinning). The doping of YBa2Cu307-δ with a wide range of dopants at the Y, Ba and Cu sites were also reported. Increased density of twin boundaries was also reported to provide only moderate improvement in flux pinning.
The formation of non-superconducting nano-particles by the introduction of dopants, such rare earth (RE), Zr, Ce, Au, etc., has also been widely evaluated. These materials can form particles by themselves or they may combine to form particles with other elements.
Pinning centers are also formed by the formation of columnar defect structures, for instance by the self-assembly of BaZrO3 into aligned or continuous structures. The most effective of these chemical based approaches has been the formation of self-assembled nano-columns formed from materials such as BaZrO3 or a combination of these nano-columns and nano-particles. Although these pinning microstructures are generally easy to prepare in small R&D samples of the RE123 superconductors, it is much more challenging to incorporate them into continuous lengths of wire. The reason is that the formation of these complex pinning structures depends on having a highly reproducible growth process. Thus, any in-homogeneities in the chemical composition, growth parameters or texture of the superconductor layer over the length or width of the production length wire can affect the precise structure or density of the pinning defects. This change in the pinning structure leads to large variations in the current that the superconductor layer can carry in the presence of magnetic fields. Thus, the performance of the HTS wire varies over its length and/or width and is thus unsuitable for applications requiring a very consistent and predictable critical current.
Additionally, the growth process needed for the formation of the self-assembled columnar microstructures is only applicable to vapor phase deposition processes. Thus, self-assembled columnar pinning microstructures cannot be incorporated into MOD-derived superconducting films.
Introducing defects into HTS wire using high energy particle irradiation has been tried using energies in the 100's of MeV to the 10's of GeV range. At these energy levels, the irradiation generally produced correlated or columnar defects structures which results in anisotropic pinning along their axis.
Although these defect structures were effective in enhancing vortex pinning in the REBCO materials in short research samples, achieving the necessary particle energy levels required expensive research accelerators are not amenable with high volume manufacturing of continuous lengths of superconducting wire. In addition, the particle currents available for the high energy particle irradiation were limited, thus requiring long irradiation times to produce a significant number of defects in the superconducting layer to provide effective pinning.
As described in U.S. Patent Publication No. 2015/0263259, irradiation with low energy particles has also been disclosed. This approach results in point defects that produce a more isotropic pinning structure however it has been focused on producing short samples using a static irradiation beam.
Thus the research efforts to develop vortex pinning microstructures that can be incorporated into the manufacture of long length HTS wires has focused on chemical techniques, described above, that attempt to replicate the combination of correlated and point defects produced by the high energy particle irradiation.